It is well known that the excessive generation, accumulation, and sparking discharge of electrostatic charges during the manufacture and use of photographic film and paper products are undesirable. Electrostatic charges may be generated in these materials by frictional contact with and separation from dissimilar materials such as transport rollers. The accumulation of static charge on film or paper surfaces can cause irregular static marking fog patterns in the emulsion layer. The presence of static charge can also lead to difficulties in support conveyance as well as the attraction of dust that can result in fog, desensitization, and other physical defects during emulsion coating. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) also can produce irregular fog patterns or “static marks” in the emulsion layer. Sheet films are especially subject to static charging during use in automated high-speed film cassette loaders and laser imagers (e.g., x-ray, graphic arts films).
One of the most widely used methods for preventing the excessive generation of electrostatic charges on photographic film and paper products is to add surface active compounds or surfactants to overcoat or other layers, which reduces the amount of charge generated on the overcoat surfaces as described above. It is also a common practice to add coating aid surfactants to overcoat layers to improve the layer thickness uniformity of the layers, especially in coating methods for the simultaneous application of two or more layers.
The polarity of the static charges formed by frictional contact on the surfaces of most gelatin-containing overcoat compositions that also contain hydrocarbon coating aid surfactants incorporated for improved coating uniformity during the coating process, is usually a positive polarity. However, when surfactants containing highly fluorinated alkyl groups in their hydrophobic ends are incorporated into overcoat compositions, the resulting static charging of the overcoat surfaces by frictional contact is reduced in its magnitude of positive polarity or becomes closer to neutral or even negative in polarity. The extent of change in charging behavior depends on the amount of fluorinated surfactant used and its molecular structure, which influences its relative effectiveness in negative charging. The composition and amount of the fluorinated surfactant incorporated in the overcoat layer or other layer, in combination with the hydrocarbon coating aid surfactants and other addenda in the overcoat layer, are selected for optimal performance of the product type under conditions of its manufacture and use. When an effective fluorinated surfactant is used at its optimum amount, the electrostatic charging propensity of the overcoat surface is minimized under those conditions of handling and transport during manufacturing and in automated film handling and exposure equipment which are most likely to cause unwanted static charge buildup and static marking.
Not all fluorinated surfactants are equally effective in exhibiting this negative charging property when present in overcoat layer compositions containing hydrocarbon coating aid surfactants and other addenda such as lubricants. The length of the fluorinated carbon chain and the total number of fluorine atoms and their relative positions on the chain, as well as the composition of other groups in the surfactant molecule, are important factors in influencing the negative charging effectiveness of the surfactant. If the number of fluorinated carbon atoms in a surfactant molecule with one or two fluorinated carbon chains is too few, the negative charging property is greatly diminished. If the number of fluorinated carbon atoms is too many, the solubility of the surfactant in water solutions is too low to be of practical use.
Nonionic fluorinated surfactants useful as coating aids and for the control of electrostatic charging in overcoat layers of photographic elements are disclosed in Chen, et al., U.S. Pat. No. 4,582,781. A combination of two surfactants for the overcoat layers of both sides of a duplitized black and white X-ray recording material, wherein one of the surfactants is a mixture of Rf—CH2CH2—S—CH(COOH)CHC(═O)HN—CH2CH2CH2N(CH3)2, and Rf—CH2CH2—S—CH(CH2COOH)C(═O)HN—CH2CH2CH2N(CH3)2 is disclosed by Adin, et al., U.S. Pat. No. 6,232,058. A black and white silver halide motion picture sound recording film overcoated with a layer containing a combination of three surfactants, including a mixture of Rf—CH2CH2—S—CH(COOH)CHC(═O)HN—CH2CH2CH2N(CH3)2, and Rf—CH2CH2—S—CH(CH2COOH)C(═O)HN—CH2CH2CH2N(CH3)2 is disclosed by Gerlach, et al., U.S. Pat. No. 5,837,440.
In the past the most readily available fluorinated surfactants which have been especially effective for adjusting static charging properties of photographic film and paper products have been those with a large fraction of perfluoro-octyl groups. Furthermore, many of the fluorinated surfactants are either perfluoro-octyl sulfonate in their original form or have structures that may degrade to a perfluoro-octyl sulfonate compound. Recent reports indicate perfluoro-octyl sulfonate may accumulate in the blood systems of humans and animals and show toxicity in laboratory animals at high chronic levels of ingestion. Therefore there is interest in identifying alternative surfactants that do not exhibit these characteristics. Fluorinated surfactants that do not break down to perfluoro-octyl sulfonate or that accumulate less than perfluoro-octyl sulfonate in the blood system of animals are desired.
Telomer-formed compounds with F(CF2CF2)x—CH2—CH2— groups cannot break down to perfluoro-alkyl sulfonate. Quantitative Structure Activity Relationships analyses based on computer software available from SRC (Syracuse Research Corporation) indicate that fluorinated surfactants with telomer-formed fluoroalkyl groups and especially groups which have six or fewer fluorinated carbons (and ethylene groups directly bonded to them) present a lower risk of bio-accumulation.
In addition, fluorinated surfactants used in overcoat layers of photographic elements must have good solubility in the coating solutions of the overcoat layers and provide control of static electric charge, without exhibiting adverse effects on the coating uniformity of the overcoat layer or the underlying image forming layers. An additional requirement is that the surfactants of the protective overcoat layer should not adversely change the photographic performance of underlying image-forming layers.
It would be desirable to provide the industry with fluorinated surfactants that not only satisfy these requirements but also are environmentally acceptable.